Highly stable atomic frequency standards are of increasing importance for a variety of applications, ranging from communication to navigation and time-transfer to tests of fundamental science. Atomic clocks are today critical components in a variety of space systems, including the Global Positioning System, communication satellites, and scientific experiments for the International Space Station. It is desirable to have improved clock stability, as well as reduced size, weight sand power usage with increased lifetime and reliability. Atomic clocks are, as a matter of principle, less susceptible to radiation effects than, for example, quartz-crystal oscillators, and are well suited for many space applications. As the need for ever more accurate and stable atomic clocks continues to grow, large laboratory primary standards are moving in the direction of laser-pumped atomic fountain clocks. At the same time, however, the need for high performance secondary standards suitable for space applications grows as well, with the added requirements of reduced size, weight and power consumption and increased reliability. These efforts require a thorough understanding of the underlying physical processes operating in the clocks, and include the use of new techniques such as laser optical pumping and laser cooling and trapping.
Atomic clocks have long employed Ramsey's method of separated oscillatory fields. The atomic system can be based on different quantum transitions, and on different types of atoms, such as, cesium (Cs) and cesium based atomic clock systems are well known. The locking of a microwave source to the cesium (Cs) transition has been achieved to provide a stable practical ticking atomic clock signal. A type of atomic clock provides a clock signal based on an atomic system where hyperfine quantum transitions are used as an internal reference for providing a Ramsey output. The Ramsey output is fed into a phase detector providing an error signal to a voltage controlled oscillator providing the clock signal that is fed into a frequency synthesizer then providing microwave excitation to the atomic system in a closed loop. A phase modulator provides a reference signal to the phase detector and provides a modulation signal to the frequency synthesizer for closed loop control.
Certain atomic clock systems have used pulsed cold atom beams that propagate through a vacuum chamber, are illuminated by laser beams and detected by state-selective laser induced fluorescence detected by photodetectors for generating the Ramsey output signal. The frequency stability of atomic clocks is usually characterized by the well-known Allan deviation equation which uses a series of fractional frequency deviation measurements. A high signal to noise ratio and a narrow clock signal are required for superior stability and clock performance. Such Ramsey atomic systems may be based upon a microwave cavity excitation or Raman processes. Raman transitions have been investigated for application to Ramsey-type atomic frequency standards. For semiconductor laser excitation of the Raman transition in a Cs thermal atomic beam, results indicate a projected short term Allan deviation of [6X10E-11]/[T]. Because of the limited interaction time for their thermal atomic beam, the Ramsey fringe width was about one kHz.
In certain cesium beam atomic clocks, the clock signal may be based on Ramsey's separated oscillatory field method and the clock stability is related to the width of the central Ramsey. When other broadening mechanisms are negligible, this fringe width is limited by the interaction time, that is the time of flight of cesium atoms through the apparatus. By employing laser cooling techniques to produce a slow, cold atomic beam, the width, that is the frequency variation, of the clock signal can be reduced, improving clock performance or, alternatively, achieve comparable performance in a more compact design. Atomic beam slowing techniques based on spontaneous emission optical forces generally suffer from transverse velocity diffusion that limits the atomic beam brightness and hence the clock signal to noise ratio. In addition, the slowing distance is often larger than the rest of the apparatus combined. There are a number of alternatives to spontaneous force atomic beam slowing, for example longitudinal atomic beam slowing with stimulated forces from a bichromatic laser field.
Improving signal to noise in a beam-type atomic clock requires not simply a large number of atoms, but a large number of atoms in the .linevert split.M=0&gt; magnetic sublevel of one of the two ground hyperfine levels. The Ramsey method in conventional Cs beam atomic clocks employs Stern-Gerlach magnets to perform the state selection and measurement, and a U-shaped microwave cavity for the separated oscillatory fields. This conventional Cs beam approach makes use of only a fraction of the initial total atomic flux because of the ground state degeneracy. In addition, the Stern-Gerlach magnets have a narrow velocity passband, and the net result is that only a couple percent of the total atomic flux contributes to the clock signal. In recent years, however, there has been significant interest in optically pumped Cs beam standards in which laser optical pumping replaces the Stern-Gerlach magnets. Such optically pumped standards are based on hyperfine and magnetic optical pumping techniques that can significantly increase the population in the .linevert split.M=0&gt; state, with a corresponding improvement in signal to noise ratio. An improved signal to noise ratio is achieved with laser optical pumping and state selective fluorescence detection replacing the Stern-Gerlach magnets.
The single laser beam magneto-optic trap (MOT) is one means to provide a cold atom beam. The MOT may be formed with a single circularly polarized laser beam from a 150 mW DBR diode laser SDL-5722 directed into a right-angle conical reflector. Polarization changes upon reflection and the linear gradient magnetic field, flipping the sign at the origin, provide the three-dimensional counter-propagating light fields of the appropriate polarizations required for the MOT. The conical reflector may be a 2.5 cm radius OFHC-copper cylinder with a diamond-machined conical inner surface and a protected gold reflective coating of rms surface roughness less than 5 nm. The gold reflective coating is protected by a transparent dielectric protective overcoat. The MOT is effective for trapping atoms at the center of the MOT and the cold atoms are forced out of the MOT along a column by the central portion of the incident laser beam, forming a low-velocity intense source (LVIS) of cold atoms in a beam. One disadvantage of conventional optical molasses or MOT-based sources, such as atomic fountains or the PHARAO space clock, is that they result in a discontinuous atomic beam at discrete pulses, and involve a plurality of orthogonal trapping laser beams. The discrete pulses disadvantageously require the use of time-segmented processing elements and local oscillators. In a conventional MOT, without a leaking dark column, the number of trapped atoms is roughly independent of the temperature, because the loading rate and collisional loss rate depend on temperature in a similar way. In the LVIS, because the loss is dominated by the transfer of atoms from the MOT to the atomic beam, increasing the operating temperature, up to a point, will increase atomic flux significantly. To further increase the atomic beam intensity, a transverse laser cooling region immediately follows the output of the apex of the cone. With a Ramsey interaction length of 15 cm, and shot noise-limited fluorescence detection of the atomic flux, assuming that we have employed laser optical pumping to the .linevert split.F=3,M=0&gt; state, a short-term Allan deviation of [3X.times.10E-14]/[T] may be achievable.
Short term and long term clock stability are of considerable importance. Long term clock stability is limited by random walk processes, typically driven by temperature variations, laser or microwave power drifts, and similar effects. With the introduction of diode lasers and atom cooling and trapping techniques, new elements are introduced that will influence long term stability of the atomic beam from a MOT. It is thus important to consider the suitability of these advanced clock concepts to space applications, including issues of operation in the space environment as well as size, weight, power requirements, and clock lifetime and reliability. Space environment related issues include the microgravity environment that poses obvious problems for an atomic fountain clock and the space radiation environment, both natural and man-made. An operating MOT has been used in the microgravity environment of parabolic trajectory airplane flights, and the PHARAO project in the development of a microgravity cold atom clock. Presently there is a development of a space-flyable MOT for evaluation on a future Space-Shuttle flight.
The use of a pulsed cold atom beam necessitates the use of intermittent laser excitation. It is desirable that the cold atom beam be continuous for continuity of the closed loop control signal. Additionally, existing atomic clocks based on laser cooling and trapping techniques suffer from large size and considerable technical complexity. These and other disadvantages are solved or reduced using the invention.